At 5.20am, police were called to reports of horses loose on State Highway 1, near Oruanui Rd.
The New Zealand Herald reports there were two horses.
"The horses posed a significant risk to motorists travelling along a main busy route in the dark, with the potential for a more serious crash," Senior Sergeant Andrew Livingstone said.
Before police arrived, a vehicle hit one of the horses.
The horses ran down Tram Rd, off State Highway 1, and officers tried to recapture them, he said.
However, the horses picked up speed and took off towards the highway.
"Unfortunately, police had to resort to fatally shooting them," Livingstone said.
"Police use every option available to us when we are called to these incidents with the aim to prevent harm to members of the public, and - where we can - the animals.
"However, due to the risks involved, in some cases, we unfortunately have to resort to serious measures."
He urged farmers and livestock owners to secure their properties.
Police were looking for the driver whose car hit one of the horses, to ensure they were OK.
